The need for non-destructive detection of defects in piping is necessary to avoid costly shut down of equipment and for ensuring the integrity of aged piping, specifically where the aged piping is carrying high pressure combustible fluids and gases that pose a significant safety or health hazard. Potential defects affecting the quality and effectiveness of piping may include, but are not limited, to flaws such as cracks, dents, and pits.
Traditionally, the non-destructive testing of piping was accomplished through systems utilizing ultrasonic piezoelectric transducers that were coupled to the surface by a fluid or gel. However, the use of a couplant creates numerous problems reducing the overall effectiveness of the system, and it is, therefore, desirable to develop and use other types of inspection systems.
As an alternative to piezoelectric transducers, techniques utilizing ultrasonic inspection of conductive surfaces have been developed. These techniques use electromagnetic acoustic transducers (EMATs) to transmit, without the use of a couplant, an ultrasonic wave into the piping.
In order to generate the ultrasonic wave, the EMAT induces eddy currents at the surface of the conductive piping under test. These induced eddy currents interact with the magnetic field produced by an electromagnet or permanent magnet located near the EMAT. The result of the interaction is a Lorentz force that acts upon the conductive piping to produce an ultrasonic wave.
Until a defect is encountered, the induced ultrasonic wave propagates through the conductive piping. The interaction between the defect encountered and the ultrasonic wave causes the propagated wave to reflect back toward the EMAT. Once the reflected ultrasonic wave reaches the EMAT, the EMAT's receiver is responsible for receiving and processing the low level reflected signals. As such, this instrumentation is susceptible to noise pick up from many sources.
In order to eliminate specific sources of noise, shielding of the EMAT is usually performed. As an example, MacLauchlan (U.S. Pat. No. 5,608,691), which is incorporated herein by reference, discloses a shield for an EMAT that has multiple alternating layers of electrically insulating and electrically conductive materials which encapsulate the EMAT in order to substantially shield the EMAT from electrostatic noise. A primary source of electrostatic noise is AM radio signals. Although other simple shields (such as a pieces of aluminum foil) attenuate both the AM radio signals and the EMAT signals, the MacLauchlan shield is capable of attenuating most of the electrostatic (AM radio) noise, while keeping the EMAT signal attenuation to a minimum.
Although the MacLauchlan shield effectively attenuates electrostatic noise, there are still other forms of noise that affect the performance of an EMAT and are not attenuated by current shielding techniques. Thus, a heretofore unaddressed need exists in the industry of reducing the unattenated noise in current EMAT design.